Various tetrafluoroethylene-perfluoro(alkyl vinyl ether) copolymers have been so far well known, and particularly tetrafluoroethylene-perfluoro(methyl vinyl ether) copolymer and tetrafluoroethylene-perfluoro(propyl vinyl ether) copolymer have been in popular use.
In the case of tetrafluoroethylene-perfluoro(methyl vinyl ether) copolymer, the copolymerizability of perfluoro(methyl vinyl ether) with tetrafluoroethylene is satisfactory, but the intermolecular interaction of side chain methoxy groups per se of the copolymer is so weak that it is difficult to obtain moldings with a satisfactory mechanical strength. That is, when compared with tetrafluoroethylene-perfluoro(propyl vinyl ether) copolymer, the mechanical strength is not better under the conditions of the same perfluoro(alkyl vinyl ether) content and the same specific melt viscosity.
In the case of tetrafluoroethylene-perfluoro(propyl vinyl ether) copolymer, on the other hand, the copolymerizability of perfluoro(propyl vinyl ether) with tetrafluoroethylene is so low that it is difficult to increase the copolymerization ratio of perfluoro(propyl vinyl ether), and thus copolymers with about 3 wt. % perfluoro(propyl vinyl ether) content are commonly used. Therefore, the copolymer has a distinguished chemical stability, but has a poor transparency due to the crystallinity.
Even if the comonomer content of perfluoro(propyl vinyl ether) in the copolymer could be increased to about 16 wt. %, the composition would become heterogeneous because of the low copolimerizability of the vinyl ether with the tetrafluoroethylene, and also the molecular weight would not be increased. Thus, it is difficult to obtain moldings with satisfactory mechanical strength and distinguished transparency.
It is well known that tetrafluoroethylene-perfluoro(ethyl vinyl ether) copolymer has distinguished transparency and flex fatigue resistance.
As disclosed in Examples of JP-B-48-20788, the copolymer with satisfactory mechanical strength cannot be obtained, when the comonomer content of perfluoro(ethyl vinyl ether) in the copolymer is not more than 3 wt. %. This means that in the case of a low comonomer content of perfluoro(ethyl vinyl ether) the intermolecular interaction of side chain ethoxy groups per se of the copolymer is so weak that no satisfactory mechanical strength can be obtained.
Also, as disclosed in Examples of JP-A-7-126329, even if the comonomer content of perfluoro(ethyl vinyl ether) is 7 wt. % and the specific melt viscosity is high, no high mechanical strength can be obtained, though a high flex fatigue resistance can be obtained. This also means that in a range of low comonomer contents of perfluoro(ethyl vinyl ether) the contribution of side chain ethoxy groups to the main chain of the copolymer is so low that no satisfactory mechanical strength can be obtained.
WO 97-07147 discloses tetrafluoroethylene-perfluoro(ethyl vinyl ether) copolymer with a comonomer content above 17 wt. %, where softening takes place with a higher comonomer content of 17 wt. %, so that no copolymer with satisfactory mechanical strength will be obtained. In this case, the copolymer is produced by emulsion polymerization as in the case of the aforementioned JP-B-48-20788, and thus the molding is colored by coloring influences due to unstable terminal groups originating from residual emulsifier and ammonium persulfate, resulting in considerable transparency reduction.
Furthermore, JP-A-2001-500906 discloses an improvement of transparency by increasing the comonomer content of perfluoro(ethyl vinyl ether) to 38 wt. % or more, but no satisfactory mechanical strength is obtained. In this case, the copolymer is produced by emulsion polymerization, and thus the afore-mentioned disadvantage is unavoidable. A terpolymer of tetrafluoroethylene-perfluoro(ethyl vinyl ether)-perfluoro(methyl vinyl ether) is also disclosed in the aforementioned JP-A-2001-500906 publication. Also, in this case, the intermolecular interaction of side chain methoxy groups per se of the terpolymer is so weak that the contribution of the third monomer perfluoro(methyl vinyl ether), even with 18 wt. % comonomer content, to an increase in the mechanical strength is not observable, or rather the mechanical strength is deteriorated.
That is, in the case of copolymer of tetrafluoroethylene with perfluoro(propyl vinyl ether), perfluoro(ethyl vinyl ether) or perfluoro(methyl vinyl ether), it is difficult to obtain distinguished transparency and mechanical strength at the same time.